The six degrees of freedom are presently measured with a floating frame type of string gauge, the centre core being an assembly of several pieces. The present tie position sting balances are designed with an internal configuration which makes every attempt to eliminate interaction between the components. However, this is not completely achieved and there is interaction between the components which varies the readings obtained from 3% to 5%.
Generally the primary frames consist of an inner rod which fastens to the model support sting, and a cylindrical outer casing which is inserted into and attaches to the model. Forces and moments are resisted by individually removable elements employing flexure pivots, these being connected between the inner rod and the outer case of the balance. The six force and moment sensing components of the balance consist of two normal force elements for determination of known force and pitching moment two side force elements for determination of side force and yawing moment, a dual axial force element, and a dual role element. The normal and side force elements are equipped with relaxation members at each end and are arranged to act in roll as a set of crossed ribbon flexures. Similar relaxation members provide compliance in the axial force direction. The rolling moment elements are provided with flexure pivots at each end which are designed to transmit pure rolling moments to the gauge section. The dual axial force element is located inside the dual roll element and transmits axial force from the outer case to the inner rod.
Variations on this typical arrangement occur depending upon the balance manufacturers, however the principal characteristic of all of these balances is that no sensing component has a primary frame roll at one time.
There are limitations associated with the present sting mounted balances, the major one being their inability to handle starting and stopping loads. When balances with adequate nominal ratings designed to overcome overloads encountered during a test run, are used, the precision of the measurements deteriorates. Usually, forces and moments which are expected to be measured are slightly less than the overloads encountered at the beginning of a test run.
The internal configuration of the known sting mounted balance is intended for classical type strain gauges that measure large displacements (20000 .mu.in/in). These strain gauges have a limited precision when compared to the new semi-conductor type strain-gauges that are 100 times more precise but are limited in displacement (5000 .mu.in/in). This leaves room for a new internal configuration of the balance specifically intended for semiconductor type strain gauges.
The internal configuration of the known balance is also extremely complicated and made of an assembly of several pieces. These pieces are all secured in place and vibration loosening can be a problem.
The internal complexity of the known balance can also lead to extreme fragility wherein overloading can have a disastrous effect.
It is also difficult to machine such internal complexity into the known balances and there is therefore a high cost of manufacture.
The type of strain gauges which are used in the known balances are, as indicated above, the large displacement type which are influenced by temperature variations encountered during a wind tunnel test run. They, therefore, have to be fitted with thermocouples.
The strain gauges used are also of 1000 ohms or more in order to obtain acceptable sensitivity and minimize overheating. The very small thickness and cross-sectional area of the sensing components combined with the poor heat conductivity associated with a multiple piece assembly lead to the necessity to use special strain gauges. Usually, standard strain gauges of 1000 ohms or more are too large to be fitted into a sting mounted balance of 0.5 inches in diameter. The type of strain gauge used in these balances must therefore also be of special design. The repairing of strain gauges in house therefore cannot be done.
To summarize, the difficulties associated with sting mounted balances which are mandatory to test axisymmetrical type projectiles, is due to their fragility about the pitching moments, side forces and yawing moments principally. Also at the beginning of a wind tunnel test run, wind start up causes symmetrical shock waves to pass momentarily over the model and these can generate overloads on the balance. The same phenomena happens at the end of the test run. These forces are recognized as starting and stopping loads and, because of these overloads, a much stronger balance has to be used. Using a much stronger balance is to the detriment of recorded measurement precision because of the higher nominal rated capacity balance. The balance of this invention has a new configuration that can tolerate starting and stopping overloads and be 300% to 500% more precise than most of the present six degrees of freedom precision balances.